The present disclosure relates to a transfer unit including an endless transfer belt, and a plurality of transfer rollers for transferring individual color toner images onto the transfer belt or a recording medium held and conveyed on the transfer belt, so as to be sequentially overlaid, and to an image forming apparatus equipped with the transfer unit.
Conventionally, various image forming apparatuses are proposed, and among them there is a tandem color image forming apparatus in which a plurality of image forming portions sequentially overlays individual color toner images so as to form a full color image. Further, the tandem color image forming apparatuses include two types: one is a direct transfer type in which toner images formed by individual image forming portions are transferred onto a recording medium conveyed on the endless transfer belt, and the other is an intermediate transfer type in which a plurality of image forming portions sequentially overlay (primarily transfer) toner images on an endless intermediate transfer belt, and then the toner images are transferred (secondarily transferred) onto a recording medium at one time.
In a monochrome mode of the tandem color image forming apparatus, in which a monochrome image is output, only black toner is used for performing image formation. In this monochrome mode, if image carriers of the image forming portions of yellow, magenta, and cyan other than black are kept in contact with the transfer belt, the transfer belt or the recording medium becomes dirty due to the contact with the yellow, magenta, and cyan image carriers, or a driving torque of the transfer belt is unnecessarily increased as a malfunction. Therefore, there is known an image forming apparatus that can switch between a contact mode and a standby mode. In the contact mode, the yellow, magenta, cyan, and black image carriers contact with some of or all the transfer rollers via the intermediate transfer belt. In the standby mode, all the transfer rollers are separated from them.
As described above, when performing a contacting or separating operation of the transfer roller, conventionally, a drive time of a drive motor for moving the transfer roller in a reciprocating manner between a contact position and a separation position is controlled so that the transfer roller is stopped at the contact position or the separation position. Therefore, a stop position of the transfer roller may vary, and hence a press contact state of the transfer roller to the photosensitive drum varies so as to affect a transfer performance of a toner image as a problem.
Accordingly, there is proposed a method of accurately stopping the transfer roller at the contact position or the separation position. For example, there is known an image forming apparatus having a structure for moving a primary transfer roller in a reciprocating manner in accordance with a rotation of a cam so that the transfer belt contacts with or separates from the photosensitive drum, in which a sensor for optically detects a rotation position of the cam.